The present invention relates generally to compression release engine braking, and more particularly to an engine braking control strategy that avoids turbine overspeed under various atmospheric conditions.
Engine compression release brakes are used to assist and supplement wheel brakes in slowing heavy vehicles such as tractor-trailers. Engine brakes are desirable because they help alleviate wheel brake overheating. As vehicle design and technology have advanced, the hauling capacity of tractor-trailers has increased, while at the same time rolling resistance and wind resistance have decreased. Thus, there is a need for advanced engine braking systems in today""s heavy vehicles.
As engine braking technology has developed, various strategies have emerged to control the magnitude of the braking horsepower. In one strategy, electronically controlled engine brake actuators allow braking horsepower to be varied by varying the timing of the blow down portion of the braking event. In other words, less than the maximum available braking horsepower can be achieved by advancing the timing of the opening of the exhaust valve, such that blow down occurs before peek pressure is achieved. Another strategy for varying engine braking horsepower is to utilize a variable geometry turbocharger, such as that taught in co-owned U.S. Pat. No. 5,813,231 to Faletti et al. That reference teaches changing the geometry of the turbocharger in order to reduce or increase the pressure in the exhaust manifold to increase or decrease engine braking horsepower, respectively. While these two and other strategies appear sound in controlling engine compression release braking horsepower to some extent, their incorporation into current engine/vehicle designs can sometimes introduce new problems.
For instance, like any device having rotating components, turbochargers are generally designed to operate under some pre-determined speed. Those skilled in the art will recognize that when a engine is not in a power mode but is in a engine braking mode, the turbine is still driven to rotate. Thus, while a turbocharger may be designed to avoid overspeed conditions at any engine powered operating condition, potential turbine overspeed must also be examined with regard to contemplated engine compression release braking under a variety of conditions. As engine braking horsepower continues to be increased with such innovations as two cycle braking and/or two event (boosted) engine braking, potential problems associated with turbine overspeed can become more pronounced.
Another factor that can influence turbine speed is related to the ambient conditions surrounding the turbine. For instance, those skilled in the art have recognized that lower ambient pressures, such as those experienced at higher altitudes, tend to cause a turbine to rotate faster. The present invention is directed to these and other problems associated with engines equipped with both turbochargers and engine compression release brakes.
In one aspect, a method of engine compression release braking with avoidance of turbine overspeed comprises the initial step of determining whether a turbine overspeed condition is present. If a turbine overspeed condition is present, then the blow down timing of the engine compression release braking is advanced.
In another aspect, an electronic control module includes a means for advancing blow down timing during engine compression release braking if a turbine overspeed condition is present.
And still another aspect, a vehicle includes a vehicle housing with an engine attached thereto. The engine includes at least one turbine and at least one engine compression release brake. An electronic control module includes a means for advancing a blow down timing during engine compression release braking if a turbine overspeed condition is present.